Requirements are often set to the antenna structures of small radio devices that often cause mutual conflicts. The antenna should be small and efficient (the efficiency of an antenna upon transmission is determined as the relation of the radiated power to the power supplied to the antenna). It should have a broad bandwidth, which covers well the whole frequency range to be used and, in addition, the antenna should be easily adjustable to the impedance of the antenna gate of the radio device. Further, the antenna should be of a solid structure, and its manufacture should be easy. A very big part of time used in the manufacture of a single antenna in serial production goes to the heating and cooling phases required by the solders so that as small a need for solders as possible would be preferable from the manufacturing-technical point of view.
An antenna type, which has very advantageous bandwidth characteristics in spite of its small size, is a folded dipole. Some recent variations in the folded dipole principle are known, for example, from the reference publications U.S. Pat. No. 5,293,176 and U.S. Pat. No. 5,796,372. However, a disadvantage of a folded dipole is that its natural feed impedance sets approximately near to 300 ohm, while most radio devices are designed taking into consideration antenna impedances of 50 ohm or 75 ohm. Connecting a folded dipole as the antenna of such a radio device requires the use of a balun or some other interface circuit, which causes additional costs in the manufacture and generally narrows the available bandwidth.
One small antenna type is also known from the reference publication US 2004/0222937 A1. An especially broad bandwidth is given as its advantage. However, the radiating part of the antenna is complex, and its several branches have not been supported very well mechanically.